(a) Field of the Invention
The present invention relates to a negative electrode for a rechargeable battery (secondary battery) including lithium metal or lithium alloy as an active material.
(b) Description of the Related Art
Rechargeable batteries using lithium metal and lithium alloy as negative electrodes show superior energy density compared with lithium-ion rechargeable battery using a graphite negative electrode. However, when a negative electrode contains lithium metal as an active, material, some drawbacks for cycling efficiency and safety must be overcome. Due to high chemical activity of lithium, a part of the lithium deposited on the negative electrode in charge process becomes inactivated, and then various lithium compounds like lithium hydroxide, lithium oxide, lithium carbonate, are formed on the electrode surface. Thus, the formed film is composed of various products. Such a non-uniform film gives a localized deposition of lithium on the negative electrode surface. Lithium inactivation induces to lower the cycling efficiency, and the localized deposition of lithium forms dendrite structure, which can cause internal short circuit. The pulverization of the lithium alloy due to a large volume change decreases charge-discharge efficiency (cycling efficiency). The volume change upon cycling must be controlled to materialize a metallic lithium electrode in commercial rechargeable batteries.
Various proposals have been made to overcome the problems for the lithium metal or alloy negative electrode. For instance, some proposals include the suppression of the dendrite growth by a stable film such as lithium fluoride on the electrode surface.
JP-A-7(1995)-302617 describes a lithium negative electrode covered with a lithium fluoride film formed by a reaction between lithium compounds existing on the lithium surface and hydrogen fluoride by means of exposing the lithium electrode to an electrolyte solution containing the hydrogen fluoride. Hydrogen fluoride is generated through a reaction between LiPF6 and a trace amount of water.
Normally, it is quite difficult to obtain a uniform surface film, because active lithium metal reacts with almost all chemical species in an electrolyte solution. The formed film consisting of various compounds is heterogeneous, which are derived from some competitive reactions on the lithium surface. As for topology, dendrite shape forms on the non-uniform surface. In the case of this method for fluoride film formation, hence, it is hardly available to attain a uniform and stable film.
JP-A-8(1996)-250108 describes the formation of lithium fluoride on the surface of a negative electrode by a reaction between a mixed gas containing argon and hydrogen fluoride and aluminum-lithium alloy. However, when another lithium compound exists on the lithium surface in advance and especially a plurality of species are present, the uniform reaction is not likely to proceed, and the uniform lithium fluoride film can be hardly formed. Accordingly, the lithium rechargeable battery with the sufficient cycle characteristics is difficult to be fabricated.
JP-A-11(1999)-288706 describes the formation of a surface film containing sodium chloride as a main component, which has a uniform (100) crystal plane preferentially oriented. In this manner, the uniform deposition and dissolution reaction of lithium suppresses the dendrite formation. Therefore, the cycling efficiency and safety of the battery could be improved. The surface film is desired to contain lithium halide; a solid solution composed of LiF and at least one of the lithium compounds such as LiCl, LiBr and LiI. Preferably, the negative electrode for the non-aqueous electrolyte battery is fabricated by dipping a lithium sheet, having a preferentially oriented (100) crystal surface and formed by rolling, into an electrolyte containing a fluorine molecule or a fluorine ion and at least one of a chlorine molecule or a chlorine ion, a bromine molecule or a bromine ion and an iodine molecule or an iodine ion for forming the solid solution film. In this art, the rolled lithium metal sheet is likely to be exposed to air, thereby forming a film on its surface due to water moisture. Suppression of the dendrite formation is insufficient because the non-uniform chemical compounds hardly give the uniform and stable film on the lithium surface.
Further, the above prior art includes the following common problems.
Although the formation of the lithium halide film on the lithium or lithium alloy layer provides suppression effect of the dendrite formation during the initial stage to some degree, the protection ability of the film decreases gradually upon cycling. This suggests that the internal stress is generated in the lithium halide film by diffusion and/or migration of lithium ions. In other words, the volume of the lithium or lithium alloy layer is changed significantly, due to the occlusion and the release of the lithium, while the volume of lithium halide layer remains unchanged. The internal stress destroys a part of the lithium halide film to lower the suppressing effect of the dendrite formation.
In view of the foregoing, an object of the present invention is to provide a negative electrode for rechargeable battery which can prevent the dendrite formation in a lithium metal negative electrode for a longer period of time and is excellent in its energy density and cycle life.
The present invention provides a negative electrode for a rechargeable battery including: a current collector, a first layer containing a conductive material to occlude and release lithium ion, the first layer formed on the current collector, a second layer containing a metal selected from lithium and lithium alloy, the second layer formed on the first layer, and a third layer containing a lithium ion conductive material, the third layer formed on the second layer.
In the present invention, the conductive material of the first layer may be replaced with conductive polymer.
In accordance with the present invention, the third layer prevents the lithium and/or the lithium alloy in the second layer from being in contact with the electrolyte and smoothly feeds the lithium to the second layer to improve the efficiency of the negative electrode. In the three-layer structure of the negative electrode, however, the third layer cannot follow the volume change of the second layer. The first layer has relaxation effect to the second layer, which shows a large volume change. That is, the first layer can occlude and release a part of the lithium incorporated in the second layer. This reduces the volume change of the second layer. Such a smaller volume change inhibits to cause pulverization.
The above and other objects, features and advantages of the present invention will be more apparent from the following description.